2017-09-13 LeetCode_282 Expression Add Operators

282. Expression Add Operators

Given a string that contains only digits 0-9 and a target value, return all possibilities to add binary operators (not unary) +, -, or * between the digits so they evaluate to the target value.

Examples: 

"123", 6 -> ["1+2+3", "1*2*3"] "232", 8 -> ["2*3+2", "2+3*2"]"105", 5 -> ["1*0+5","10-5"]"00", 0 -> ["0+0", "0-0", "0*0"]"3456237490", 9191 -> []

Credits:
Special thanks to @davidtan1890 for adding this problem and creating all test cases.

solution:

#include <stack>

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class Solution {

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public:

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    void divides(vector<string>& ans, string& num, int target, string str, int d, bool flag, stack<long long> nums, stack<char> syms, int state) {

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        if (state == 0) {

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            nums.top() = nums.top()*10+num[d-1]-'0';

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        } else if (state == 1 || state == 2) {

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            while (syms.size()) {

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                long long x = nums.top(); nums.pop();

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                if (syms.top() == '+') nums.top() += x;

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                else if (syms.top() == '-') nums.top() -= x;

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                else nums.top() *= x;

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                syms.pop();

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            }

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            if (state == 1) syms.push('+');

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            else syms.push('-');

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            nums.push(num[d-1]-'0');

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        } else {

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            while (syms.size() && syms.top() == '*') {

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                long long x = nums.top(); nums.pop();

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                nums.top() *= x; syms.pop();

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            }

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            syms.push('*');

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            nums.push(num[d-1]-'0');

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        }

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        if (d == num.length()) {

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            while (syms.size()) {

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                long long x = nums.top(); nums.pop();

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                if (syms.top() == '+') nums.top() += x;

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                else if (syms.top() == '-') nums.top() -= x;

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                else nums.top() *= x;

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                syms.pop();

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            }

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            if (target == nums.top()) ans.push_back(str);

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            return;

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        }

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        if (flag) divides(ans, num, target, str+num[d], d+1, true, nums, syms, 0);

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        divides(ans, num, target, str+"+"+num[d], d+1, num[d]-'0', nums, syms, 1);

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        divides(ans, num, target, str+"-"+num[d], d+1, num[d]-'0', nums, syms, 2);

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        divides(ans, num, target, str+"*"+num[d], d+1, num[d]-'0', nums, syms, 3);

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    }

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    vector<string> addOperators(string num, int target) {

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        vector<string> ans;

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        if (num.length() == 0) return ans;

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        string str; str += num[0];

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        stack<long long> nums; nums.push(0);

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        stack<char> syms;

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        divides(ans, num, target, str, 1, num[0]-'0', nums, syms, 0);

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        return ans;

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    }

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};